1. Field of the Invention
The invention relates to measurement cells and, more particularly, to a heatable flow-through measurement cell for a gas analyzer.
2. Description of the Related Art
EP 1 767 922 A2 discloses a flow-through measurement cell comprising a tube, which is terminated at each end with a respective heatable end piece and a radiolucent window held therein. The measurement gas is supplied at one end of the tube and discharged at the other, or is preferably supplied in the center of the tube and discharged at both ends of the tube.
U.S. Pat. No. 4,205,550 discloses a flow-through measurement cell comprising an inner tube that is made of steel or aluminum and is terminated at each end with a respective end piece, which is provided with cooling fins, and a radiolucent window held therein. The inner tube is surrounded coaxially by an outer tube made of steel or aluminum such that a gap is formed. Each tube contains openings that lie diametrically opposite one another, where the passage of combustion gases to be analyzed through the measurement cell is enabled or blocked depending on the rotated position of the outer tube with respect to the inner tube.
JP 59-035 131 AA discloses a flow-through measurement cell that is made of glass and is integrated in a heatable metal block with the inclusion of a thermally conductive material that absorbs different thermal expansions, such as silver solder.
JP 57-163 845 AA likewise discloses a through-flow measurement cell that is integrated in a heatable metal block.
In contrast to in-situ gas analysis, extractive gas analysis involves extracting the gas to be analyzed from a process and conducting the extracted gas through a flow-through measurement cell, where it is analyzed by spectroscopy, non-dispersive infrared (NDIR) lasers, etc. Here, it is often necessary to take measurements at higher temperatures to prevent condensation of water in the extraction lines and the measurement cell or undesirable reactions. The measurements can be taken at temperatures of up to 200° C. using a heated measurement cell. The measurement result is dependent on the temperature of the measurement gas. As a result, the temperature of the measurement gas along the measurement section formed by the measurement cell has to be location-independent to within a few Kelvin.
In order to achieve the required temperature resistance and corrosion resistance, i.e., in the case of aggressive measurement gases, the material typically used for the inner wall of the measurement cell is high-grade steel. High-grade steel has a thermal conductivity that is relatively low for metals, however, and therefore undesirable temperature gradients can arise when heating the measurement cell, i.e., in the case of elongated measurement cells in the order of magnitude of one meter. To date, the formation of temperature gradients has been reduced, for example, by the uniform application of heating tapes to the outer side of the measurement cell, and also if appropriate by breaking the heating down into a plurality of heating circuits that are controlled separately. It is also known to arrange the measurement cell in a circulating air furnace, which makes uniform heating possible.
The expenditure in terms of instrumentation or apparatus associated with the known solutions is relatively high, however.